diff options
Diffstat (limited to 'Documentation/mm/transhuge.rst')
| -rw-r--r-- | Documentation/mm/transhuge.rst | 34 |
1 files changed, 9 insertions, 25 deletions
diff --git a/Documentation/mm/transhuge.rst b/Documentation/mm/transhuge.rst index 216db1d67d04..ec3dc5b04226 100644 --- a/Documentation/mm/transhuge.rst +++ b/Documentation/mm/transhuge.rst @@ -117,31 +117,15 @@ pages: - ->_refcount in tail pages is always zero: get_page_unless_zero() never succeeds on tail pages. - - map/unmap of the pages with PTE entry increment/decrement ->_mapcount - on relevant sub-page of the compound page. - - - map/unmap of the whole compound page is accounted for in compound_mapcount - (stored in first tail page). For file huge pages, we also increment - ->_mapcount of all sub-pages in order to have race-free detection of - last unmap of subpages. - -PageDoubleMap() indicates that the page is *possibly* mapped with PTEs. - -For anonymous pages, PageDoubleMap() also indicates ->_mapcount in all -subpages is offset up by one. This additional reference is required to -get race-free detection of unmap of subpages when we have them mapped with -both PMDs and PTEs. - -This optimization is required to lower the overhead of per-subpage mapcount -tracking. The alternative is to alter ->_mapcount in all subpages on each -map/unmap of the whole compound page. - -For anonymous pages, we set PG_double_map when a PMD of the page is split -for the first time, but still have a PMD mapping. The additional references -go away with the last compound_mapcount. - -File pages get PG_double_map set on the first map of the page with PTE and -goes away when the page gets evicted from the page cache. + - map/unmap of PMD entry for the whole compound page increment/decrement + ->compound_mapcount, stored in the first tail page of the compound page; + and also increment/decrement ->subpages_mapcount (also in the first tail) + by COMPOUND_MAPPED when compound_mapcount goes from -1 to 0 or 0 to -1. + + - map/unmap of sub-pages with PTE entry increment/decrement ->_mapcount + on relevant sub-page of the compound page, and also increment/decrement + ->subpages_mapcount, stored in first tail page of the compound page, when + _mapcount goes from -1 to 0 or 0 to -1: counting sub-pages mapped by PTE. split_huge_page internally has to distribute the refcounts in the head page to the tail pages before clearing all PG_head/tail bits from the page |